| Due to the characteristic of low boiling point, organic fluid can obtain high vapor pressure at medium and low temperature and drive the expander. By comparison with steam Rankine cycle, the organic Rankine cycle (ORC) can react well to low environment temperature, avert from being freezing in winter, easier realization of efficient turbomachinery at low power, and avoid problems such as low average heat absorption temperature and low cycle efficiency associate with superheat by using dry fluids. Solar irradiation has the essence of strong dispersion, low energy flow density and easy conversion into medium and low temperature heat source. It will be good application prospect by combining solar energy with the ORC. But it has challenges of low power efficiency and large irreversible losses at the same time.The single stage ORC is widely applied because its structure is relatively simple. However, it is only suitable in case of small temperature difference between the heat source and sink. Adopting single stage ORC in large temperature difference between the heat source and sink will result in multistage expansion and a complicated turbine. The positive displacement expander is not applicable either, because the built-in volume ratio is too small. The above problems can be solved by adopting cascade system. The cascade Rankine system is promising because thermal power efficiency can be improved and the design of turbine is easier in large temperature difference between the heat source and sink. However, relevant research is limited at present.The application of cascade system in different situations is studied in this paper, the specific content and innovation points are as follows:(1) A cascade system adopted in solar energy and liquefied natural gas (LNG) hybrid system is proposed. Energy from solar collectors drives the evaporation of working fluid in the top cycle (Ⅰ). The heat released by cycle I facilitates the evaporation of working fluid in the bottom cycle (Ⅱ). LNG is the cold source of cycle II. The system is optimized with respect to a new index, i.e., the equivalent efficiency, which denotes the ratio of the extra electricity produced by adding collectors and the received solar irradiation. Its advantages are clarified in comparison with existing indices like thermal efficiency, cold energy efficiency and exergetic efficiency. The effects of working fluids on the system performance are investigated. Flat plate and evacuated tube collectors are exemplified. It is shown that the equivalent efficiency is the most suitable index to embody the effectiveness of combination of solar collectors and LNG. A maximum equivalent efficiency of 5.99% can be obtained on the use of isopentane/R125 and evacuated tube collectors. The volume ratio (rv) for each expander in the cascade ORC is much smaller than that in single-stage ORC, thereby resulting in easier design and manufacture of the expanders.(2) A cascade system adopted in parabolic trough collector (PTC) power system is proposed. The top and the bottom are steam Rankine cycle and ORC. Particularly, screw expander (SE), which is characterized by good applicability in power conversion with steam-liquid mixture, is employed in the top cycle. Steam is generated directly in the PTC and expands in SE. The heat released by steam condensation is used to drive the ORC. This type of solar thermal power system has the advantages of avoidance of superheated steam, moderate operating temperature and pressure, low technical requirements in heat collection and storage, and suitableness for distributed power generation. Simulation of the system performance is conducted on the use of ten ORC fluids. Four hot/cold side temperatures of 473/313 K,473/293 K,523/313 K and 523/293 K are exemplified. The results indicate the ORC evaporation temperature corresponding to theoretical maximum solar power efficiency fails to provide a pressure ratio (rp) that matches the SE built-in rp. Off-design operation of the SE is recommended for the purpose of higher system efficiency and simpler ORC turbine. Efficiency of 13.68-15.62% for the proposed system can be achieved.(3) An approximate off design model based on the existing experimental data is proposed, which reveals variation of actual SE isentropic efficiency with operating rp. Optimization of the novel solar electricity generation system using cascade Rankine cycle and steam SE is carried out based on the model. The results indicate that maximum solar thermal power efficiency of 13.74-15.45% can be achieved when built-in rv and beam solar radiation (Gb) are 5 and 800 W/m2, respectively. Maximum efficiency of 13.12-15.11% is achieved when built-in rv is 3.5. The effects of hot side temperature and Gb on solar thermal power efficiency are analyzed. The adverse impact of lower built-in rv on power efficiency is limited because of good SE part-load behavior.(4) An indicator, namely equivalent hot side temperature (TEHST) is proposed. TEHST is derived from ideal thermodynamic process, but can denote the efficiency of irreversible ORC. Study on 17 fluids shows that on the given operating conditions, fluid of higher TEHST generally offers higher ORC efficiency. This relationship is stronger and more universal than those established with respect to the critical temperature, boiling point, Jacobs number and Figure of Merit. An ORC model by the method of error transfer and compensation is further built, in which the efficiency is quantitatively correlated with TEHST. Unlike the conventional ORC efficiency model, this one consists of thermodynamic parameters on the liquid/vapor curve and is independent on fluid properties at superheated state, and hence is more convenient. It has high accuracy and the relative deviation of the estimated efficiency from that calculated by the conventional model is from -0.7% to 3.4%. The novel model is applied for the thermodynamic performance analysis of a new fluid of HFO1336mzzZ based on the phase equilibrium data. The results indicate HFO1336mzzZ is more efficient than R245fa on the conditions of high evaporation temperature and low pump efficiency.(5) The key units of solar cascade Rankine cycle such as heat pipe evacuated-tube solar collectors and ORC system are constructed and experimentally tested. The dynamic performance of the system in different heat source and sink is studied and the collector efficiency curve is fitted. The variations of scroll expander efficiency, net power output, ORC efficiency are analyzed. The optical efficiency at normal incidence and the first heat loss coefficient of the collector are 0.686 and 2.1 W·m-2 ·℃-1, respectively. The net power output of 710W and ORC efficiency of 4.3% can be achieved when conduction oil and condensation temperatures are 140℃ and 20℃, respectively. Maximum thermal power efficiency of the system is 1.9%. |
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